Wave soldering method and system used for the method

ABSTRACT

In a method of soldering a work-piece in a state where the work-piece contacts a wave molten solder, a soldering process is performed while controlling an immersion depth of the work-piece into the wave molten solder to be constant. The immersion depth control is performed by changing one of a height position of the work-piece and a wave height of the wave molten solder based on a displacement amount of the work-piece in a height direction thereof. Accordingly, the soldering process can be performed with high quality even when the work-piece is thermally warped.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of Japanese PatentApplications No. 10-17992, filed on Jan. 14, 1998, and No. 10-365261filed on Dec. 22, 1998, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a wave soldering method for soldering on areflow side of a work-piece by jetting molten solder onto the reflowside, and a system used for the wave soldering method.

2. Description of the Related Art

Conventionally, a printed circuit board is soldered by a wave solderingmethod to improve processing efficiency. A wave soldering system 9coventionally used for the wave soldering method is shown in FIG. 11.The wave soldering system 9 includes two solder baths 93, 94. A fluxcoating device 91 for coating flux onto a reflow side of a printedcircuit board 8 and a pre-heater 92 for heating the printed circuit boad8 are disposed on an upstream side of the solder bath 93. A transferunit 93 is disposed above these parts to sequentially transfer theprinted circuit board 8.

When a soldering process is carried out by the wave soldering system 9,the printed circuit board 8 is sequentially transferred by the transferunit 95 to pass through above the flux coating device 91, the pre-heater92, and the solder baths 93, 94. Accordingly, the back face (reflowside) of the printed circuit board 8 is soldered. Specifically, in thesolder baths 93, 94, as shown in FIG. 12, molten solder 7 is jettedupward from a wave nozzle 930 which is disposed within the solder bath93, using a solder circulating unit 97. The printed circuit board 8 istransferred with the reflow side 81 contacting the molten solder 7.

In the conventional method and system described above, however, theprinted circuit board 8 can be warped by heat transmitted from themolten solder 7. When the printed circuit board 8 is thermally warped,the following deficiencies arise. First, as shown in FIG. 13, when theprinted circuit board 8 is largely warped by the heat, the position ofthe printed circuit board 8 where the molten solder 7 is jetted thereonmay be go down. In this case, an immersion depth of the printed circuitboard 8 into the molten solder 7 is unnecessarily increased.Accordingly, bridges, solder short-circuited parts, and the like areformed on the printed circuit board 8, and as shown in FIG. 14, solderclimbing deficiency such that the molten solder 7 reaches the surface ofthe printed circuit board 8 on an opposite side of the reflow side fromits side portion arises. Further, when the printed circuit board 8 iswarped so that a circumference portion thereof escapes upward as shownin FIG. 15, the circumference portion may not contact the molten solder7, thereby causing a non-soldered portion of the printed circuit board8.

Various types of wave soldering methods and systems have been proposedto stably perform wave soldering. For example, JP-A-7-131143 discloses amethod for stabilizing a soldered sate by accurately detecting a heightof jetted molten solder. This kind of method for controlling a moltensolder level is also disclosed in JP-A-2-37964.

However, the deficiencies caused by thermal deformation of the printedcircuit board itself cannot be solved by these conventional methods.Therefore, even when the height of the jetted molten solder is preciselycontrolled, it is difficult to prevent the solder climbing deficiencyand the occurrence of non-soldered portion.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems. Anobject of the present invention is to provide a wave soldering methodfor soldering on a work-piece and a wave soldering system used for themethod capable of performing a soldering process with high quality evenwhen the work-piece is thermally warped.

According to the present invention, in a method for soldering on areflow side of a work-piece by bringing the work-piece into contact witha wave molten solder, a soldering process is performed while controllingan immersion depth of the work-piece into the wave molten solder to beconstant. The immersion depth of the work-piece can be controlled byadjusting one of a height position of the work-piece and a wave heightof the wave molten solder based on a displacement amount of thework-piece. Accordingly, even when the work-piece is thermally warped,the immersion depth can be controlled to be constant. As a result, thesoldering process can be performed with high quality.

The displacement amount can be detected by a displacement sensor servingas a displacement device. When the displacement sensor is disposed abovea contact portion between the work-piece and the wave molten solder, theimmersion depth can be detected directly by the displacement amount.Therefore, the one of the height position of the work-piece and the waveheight of the wave molten solder can be controlled in response to thedisplacement amount. When the displacement amount is detected at aplurality of points of the work-piece, a warp amount of the work-pieceis computed based on the displacement amount and the one of the heightposition of the work-piece and the wave height of the wave molten solderis controlled based on the warp amount.

In these case, the wave height of the wave molten solder can be directlydetected by a wave height detecting unit. At that time, the one of theheight position of the work-piece and the wave height of the wave moltensolder is controlled based on the displacement amount of the work-pieceand the wave height detected by the wave height detecting unit.Accordingly, the immersion depth control can be performed moreprecisely. Further, even when the wave height of the wave molten solderfluctuates, the immersion depth control can quickly response to thefluctuation.

The displacement amount can be detected using a light emitting devicewhich is disposed on a side of a contact portion between the work-pieceand the wave molten solder for emitting a measurement light onto a firstirradiation position of one of the wave molten solder and thework-piece. In this case, the measurement light is reflected at thefirst irradiation position and then is reflected a second irradiationposition of another one of the work-piece and the wave molten solder asa reflected light. The one of the height position of the work-piece andthe wave height of the wave molten solder can be controlled so that anoptical path of the reflected light becomes constant. Otherwise, the oneof the height position of the work-piece and the wave height of the wavemolten solder can be controlled so that an interval between the firstand second irradiation positions becomes constant.

In this case, likewise, the wave height can be directly detected by thewave height detecting unit so that the immersion depth control isperformed more precisely. It is preferable that the measurement light isa laser having favorable directivity, resulting in accurate immersiondepth control. It is preferably that the measurement is a pulsed light.Accordingly, a change in the optical path of the reflected light or thelike can be stepwise detected, resulting in high responsibilitycharacteristics of the immersion depth control.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become morereadily apparent from a better understanding of the preferredembodiments described below with reference to the following drawings.

FIG. 1 is a schematic view showing a constitution of a wave solderingsystem in a first preferred embodiment of the present invention;

FIG. 2 is a schematic view for explaining an immersion depth of awork-piece in the first embodiment;

FIG. 3 is an explanatory view indicating movements of displacementsensors and the work-piece in the first embodiment;

FIG. 4 is an explanatory view indicating movements of a displacementsensor and the work-piece in a second preferred embodiment;

FIG. 5 is an explanatory view showing an optical path of measurementlight in a third preferred embodiment;

FIG. 6 is an explanatory view showing an optical path of measurementlight in a fourth preferred embodiment;

FIG. 7 is an explanatory view for explaining a control method of animmersion depth in the fourth embodiment;

FIG. 8 is an explanatory view showing an optical path of measurementlight and an arrangement of a CCD camera in a fifth preferredembodiment;

FIG. 9 is a schematic view showing an image of irradiation positions inthe fifth embodiment;

FIG. 10 is a schematic view showing a constitution of a wave solderingsystem in a sixth preferred embodiment;

FIG. 11 is a schematic view showing a constitution of a wave solderingsystem according to a prior art;

FIG. 12 is a schematic view for explaining a wave soldering methodaccording to the prior art; and

FIGS. 13 to 15 are explanatory views showing soldering deficiencies inthe prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

As shown in FIG. 1, a basic structure of a wave soldering system 1 in afirst preferred embodiment is as follows. The wave soldering system 1includes a solder bath 10, a wave device 12 for jetting the moltensolder 7 by sending the molten solder 7 to the wave nozzle 11, and atransfer unit 2 for transferring a work-piece 8 while making thework-piece 8 coact the molten solder 7. The solder bath 10 stores moltensolder 7 therein and has a wave nozzle 11 for jetting the molten solder7. The wave soldering system 1 further includes three displacementsensors 3 respectively for detecting displacement amounts of thework-piece 8 in a height direction, i.e., in a direction perpendicularto a work-piece surface, a moving unit 4 for moving the height positionof the work-piece 8 in a vertical direction in response to thedisplacement amounts of the work-piece 8, and a control unit forcontrolling the moving unit 4.

Next, the structure and operation of the wave soldering system 1 will beexplained in more detail.

The work-piece 8 is a printed circuit board in this embodiment and isheld by the transfer unit 2 at a reflow side 81 thereof. The transferunit 2 includes a pair of chucks 21 extending downward from a main part20. The work-piece 8 is held by the chucks 21. Further, the transferunit 2 is disposed in a slightly inclined state to be movable in adirection indicated by arrow A (direction A) in FIG. 1. The threedisplacement sensors 3 are fixed to the main part 20 of the transferunit 2 to detect the displacement amounts of the work-piece 8 in theheight direction. Accordingly, as indicated by broken lines in FIG. 3,the displacement sensors 3 move together with the transfer unit 2 andthe work-piece 8 in direction A. Each of the displacement sensors 3 iselectrically connected to the control unit 5 to sequentially senddetecting data thereto.

The transfer unit 2 has the moving unit 4 above the main part 20thereof. The moving unit 4 is composed of a servo motor in thisembodiment, and is operated by a moving unit controller 40. The movingunit controller 40 is, as shown in FIG. 1, electrically connected to thecontrol unit 5 and controls the moving unit 4 based on indicationsprovided by the control unit 5. The control unit 5 includes aninstrument amplifier 51 and an operation part 52. The instrumentamplifier 51 executes an amplifying processing and the like of thedetection data outputted from the displacement sensors 3, and theoperation part 52 calculates a warp amount (warped shape) of thework-piece 8 based on the data processed by the instrument amplifier 51and gives indications to the moving unit controller 40. The operationpart 52 is composed of an A/D converter, a warp operation unit, acontrol signal producing unit, and a control signal output unit, whichare not shown.

A warp operation in this embodiment is executed by interpolating acircular arc using the detection data from the displacement sensors 3.Further, in the warp operation, compensation is provided by experimentaldata including a change in the work-piece warp, which is caused by theheat of the molten scolder 7, with respect to elapsed time. Variousconditions such as ambient temperature, seasons, material and acoefficient of thermal expansion of the work-piece, temperature of themolten solder 7, and the like are reflected in the experimental data.

The solder bath 10 is, as shown in FIG. 1, disposed so that the wavenozzle 11 opens upward, and guide portions 13, 14 for facilitating theflow of the molten solder 7 are disposed around the wave nozzle 11. Thewave nozzle 11 is connected to the wave device 12 at a base portionthereof. The wave device 12 includes a solder circulating unit 122 whichis rotated by a motor 121.

When a soldering process is carried out to the work-piece 8 using thewave soldering system 1 described above, as shown in FIG. 1, the moltensolder 7 is jetted out and the work-piece 8 is transferred by thetransfer unit 2 in direction A while contacting the molten solder 7. Atthat time, the immersion depth of the work-piece 8 into the moltensolder 7 is controlled to be constant. As shown in FIG. 2, when it isassumed that the molten solder 7 is jetted without being intercepted bythe work-piece 8, the immersion depth represents a length between pointB1 on the uppermost surface of the molten solder 7 and point B2 on thereflow side 81, in a direction perpendicular to the reflow side 81.

The control of the immersion depth is performed by adjusting the heightposition of the work-piece 8 based on the warp amount of the work-piece8. Specifically, the warp amount of the work-piece 8 is calculated bythe control unit 5 using the detection data of the displacement amountsat three positions of the work-piece 8, respectively detected by thedisplacement sensors 3. Next, the moving unit 4 is moved in the verticaldirection by the control unit 5 through the moving unit controller 40according to the warp amount. As a result, the height position of thework-piece 8 is moved together with the transfer unit 2 in the verticaldirection according to the warp amount of the work-piece 8. Because thewarp amount of the work-piece 8 is always detected by the displacementsensors 3 moving together with the work-piece 8, the height positionadjustment of the work-piece 8 in the vertical direction is accuratelyperformed to correspond to the continuous change in the warp amount ofthe work-piece 8, so that the immersion depth can be kept constant.Therefore, in this embodiment, even when the work-piece 8 is thermallywarped, the solder climbing deficiency and occurrence of non-solderedportions are sufficiently prevented, realizing a high quality solderingprocess.

In this embodiment, the displacement amount of the work-piece 8 isdetected in the height direction perpendicular to the work-piecesurface, and the height position of the work-piece 8 is adjusted by themoving unit 4 in the vertical direction; however, the directions inwhich the displacement amount is detected and in which the heightposition is adjusted are changeable. For example, the displacementamount can be detected in the vertical direction, and the heightposition can be adjusted in the height direction.

(Second Embodiment)

As shown in FIG. 4, in a second preferred embodiment, a displacementsensor 32 is employed in place of the displacement sensors 3 in thefirst embodiment. The displacement sensor 32 is fixed at a positionabove the contact portion between the molten solder 7 and the work-piece8 and detects a displacement amount of the work-piece 8. The detecteddisplacement amount is used as control date, and the work-piece 8 ismoved in the vertical direction according to the control data. A controlunit in this embodiment executes PID control using a proportionator (P),an integrator (I), and differentiator (D). The other features are thesame as those in the first embodiment.

In this embodiment, the work-piece 8 is moved in the vertical directiondirectly based on the displacement amount detected by the displacementsensor 32. Accordingly, the immersion depth of the work-piece 8 iscontrolled to be constant as well. In this embodiment, because thedisplacement sensor 32 is fixed at the position described above, thestructure is simplified as compared to that in the first embodiment, andthe operation in the control unit 5 is also simplified. The othereffects are the same as those in the first embodiment.

(Third Embodiment)

In a third preferred embodiment, a method for controlling the work-pieceimmersion depth is different from that in the first embodiment. Thecontrol of the immersion depth in the third embodiment is executed asfollows. As shown in FIG. 5, a wave soldering system in this embodimentincludes a light emitting device 65 and a light intercepting device 66on a forward side of the contact portion between the work-piece 8 andthe molten solder 7, in addition to the transfer unit 2, thedisplacement sensors 3, the control unit 5, and the like employed in thefirst embodiment. The light emitting device 65 is a He—Ne laser emittingdevice which emits He—Ne pulsed laser as measurement light 61.

The measurement light 61 emitted from the light emitting device 65 isprojected the molten solder 7, and is reflected by the molten solder 7toward the processes face 81. The reflected light is further reflectedby the reflow side 81 as reflected light 62, and the reflected light 62enters the light intercepting device 66. Because the reflected light 62is pulsed light, the irradiation of the reflected light 62 onto thelight intercepting device 66 is intermittently performed. Accordingly,the light intercepting device 66 stepwise knows the deviation of theoptical path of the reflected light 62. Therefore, the work-piece 8 ismoved in the vertical direction to modify the deviation. That is, theheight position of the work-piece 8 is adjusted in the verticaldirection so that the optical path of the reflected light 62 becomesconstant. As a result, the immersion depth of the work-piece 8 can becontrolled to be constant. The height position of the work-piece 8 inthe vertical direction is changed substantially in the same manner as inthe first embodiment. The other features and effects are the same asthose in the first embodiment.

(Fourth Embodiment)

In a fourth preferred embodiment, the control of the work-pieceimmersion depth is not performed by moving the work-piece 8 in thevertical direction, but by controlling a wave height of the moltensolder 7 as shown in FIG. 6. That is, as shown in FIG. 7, a wavesoldering system in the fourth embodiment includes a control unit 125for controlling the solder circulating unit 122 and for controlling thewave height of the molten solder 7. The wave soldering system dispenseswith the displacement sensors and the moving unit used in the first tothird embodiments. The other features are the same as those in the thirdembodiment.

The control unit 125 is connected to the motor 121 for operating thesolder circulating unit 122, and is composed of a controller 126 and anoperation unit 127. The operation unit 127 is electrically connected tothe light emitting device 65 and the light intercepting device 66similar to those in the third embodiment. Accordingly, the optical pathof the reflected light 62 is determined in the operation unit 127.

The light emitting device 65 and the light intercepting device 66 may bearranged as shown in FIG. 7 so that the measurement light 61 and thereflected light 62 intersect one another, and may be arranged as shownin FIG. 6 so that the lights 61, 62 become parallel to one another.These arrangements can be flexibly set according to the jet shape of themolten solder 7 and the like. In this embodiment, the wave height of themolten solder 7 is controlled in the vertical direction so that theoptical path of the reflected light 62 becomes constant. Therefore, theimmersion depth of the work-piece 8 can be controlled without using themoving unit and the like. The other effects are the same as those in thethird embodiment.

(Fifth Embodiment)

In a fifth preferred embodiment, in place of the control utilizing theoptical path of the reflected light 62, the immersion depth iscontrolled by controlling irradiation position C of the measurementlight 61 onto the molten solder 7 and irradiation position D of thereflected light onto the reflow side 81 to be constant.

Specifically, as shown in FIG. 8, the light emitting device 65 foremitting He-Ne laser as in the third embodiment and a CCD camera 69 forprocessing images of the irradiation positions C, D as image data aredisposed on the forward side of the contact portion between the reflowside 81 of the work-piece 8 and the molten solder 7. The CCD camera 69is connected to an image processing apparatus which is not shown, andproduces an image 690 as shown in FIG. 9. Further, the moving unit andthe control unit substantially the same as those in the first and thirdembodiments are provided to adjust the work-piece in the heightdirection. In this case, the work-piece 8 is moved in the verticaldirection by the moving unit and the control unit so that an intervalbetween the irradiation positions C, D become constant. Accordingly, theimmersion depth is controlled to be constant. In this case, the waveheight of the molten solder 7 can be controlled so that the intervalbetween the irradiation positions C, D becomes constant. The othereffects are the same as those in the third embodiment.

(Sixth Embodiment)

In a sixth preferred embodiment, as shown in FIG. 10, the control unit 5of the wave soldering system 1 includes a wave height detecting device55. The wave height detecting device 55 is composed of a CCD camera formonitoring a jetted top portion of the molten solder 7 and iselectrically connected to the operation part 52 of the control unit 5.The operation part 52 additionally includes a wave height operationunit, in addition to the A/D converter, the warp operation unit, thecontrol signal producing unit, and the control signal output unit. Thewave height operation unit computes the wave height of the molten solder7 based on signals from the CCD camera. Then, the control signalproducing unit gives control instructions to the moving unit controller40 based on the warp amount and the wave height outputted from the warpoperation unit and the wave height operation unit. In this embodiment,although the CCD camera is used as the wave height detecting device 55so as to detect the wave height by processing the images, other types ofdetectors such as an eddy current detector capable of directly detectingthe wave height may be adopted as the wave height detecting device 55.The other features are the same as those in the first embodiment.

In this embodiment, not only the warp amount of the work-piece 8 butalso the wave height of the molten solder 7 is considered to control thework-piece height. Therefore, even when the wave height of the moltensolder 7 fluctuates, the work-piece height is appropriately controlledin response to the fluctuation of the wave height. As a result, theimmersion depth of the work-piece 8 can be controlled precisely morethan that in the first embodiment. The other effects are the same asthose in the first embodiment. In this embodiment, although the waveheight control device 55 is adopted to the wave soldering system 1 inthe first embodiment, it may be adopted to any one of the systems in thesecond to fifth embodiments. In either case, the immersion depth can beprecisely controlled.

While the present invention has been shown and described with referenceto the foregoing preferred embodiments, it will be apparent to thoseskilled in the art that changes in form and detail may be made thereinwithout departing from the scope of the invention as defined in theappended claims.

What is claimed is:
 1. A wave soldering system for soldering on awork-piece by a wave molten solder, the system comprising: a solder bathfor storing a molten solder therein; a wave device equipped with thesolder bath for jetting the molten solder as the wave molten solder fromthe solder bath onto the work-piece; a transfer unit disposed above thesolder bath for transferring the work-piece while bringing thework-piece into contact with the wave molten solder; a displacementdetecting device for detecting a warp amount of the work-piece which iscaused by heat, the warp amount being detected from displacement amountsof a plurality of points of the work-piece in a height directionperpendicular to a work-piece surface; and an adjustment device forcontrolling an immersion depth of the work-piece into the wave moltensolder to be constant by adjusting one of a height position of thework-piece and a wave height of the wave molten solder in the heightdirection based on the warp amount of the work-piece.
 2. The system ofclaim 1, wherein: the displacement detecting device includes adisplacement sensor disposed above the work-piece on a side opposite thesolder bath and detects the height position of the work-piece in theheight direction; and the adjustment device includes a moving unit formoving the work-piece, and a moving unit controller for controlling themoving unit based on the height position detected by the displacementsensor.
 3. The system of claim 2, wherein the displacement sensor isdisposed above a contact portion between the work-piece and the wavemolten solder.
 4. The system of claim 2, wherein the displacementdetecting device includes a plurality of displacement sensors disposedabove the work-piece and fixed to the transfer unit for detecting thedisplacement amount at the plurality of points of the work-piece.
 5. Thesystem of claim 4, wherein the displacement amount is detected at leastat three points of the work-piece.
 6. The system of claim 2, furthercomprising a wave height detecting unit for detecting the wave height ofthe wave molten solder, wherein the moving unit moves the work-piecebased on the wave height detected by the wave height detecting unit andthe height position detected by the displacement sensor.
 7. The systemof claim 1, wherein: the displacement detecting device includes a waveheight detecting unit for detecting the wave height of the wave moltensolder; and the adjustment device includes a wave device controller forcontrolling the wave device to adjust the wave height of the wave moltensolder based on the wave height detected by the wave height detectingunit.
 8. The system of claim 1, wherein: the displacement detectingdevice includes a light emitting device disposed on a side of a contactportion between the wave molten solder and the work-piece for emitting ameasurement light onto an irradiation position of one of the wave moltensolder and the work-piece so that the measurement light is reflected bythe wave molten solder and the work-piece, respectively, as a reflectedlight, and a light intercepting device disposed on the side of thecontact portion for receiving the reflected light to detect an opticalpath of the reflected light, the irradiation position being adjacent tothe contact portion and facing another one of the wave molten solder andthe reflow side; and the adjustment device adjusts one of a heightposition of the work-piece and a wave height of the wave molten solderso that the optical path of the reflected light becomes constant.
 9. Thesystem of claim 8, further comprising a wave height detecting unit fordetecting the wave height of the wave molten solder, wherein theadjustment device moves the work-piece based on the wave height detectedby the wave height detecting unit.
 10. The system of claim 1, wherein:the displacement detecting device includes a light emitting devicedisposed on a side of a contact portion between the wave molten solderand the work-piece for emitting a measurement light onto a firstirradiation position of one of the wave molten solder and the work-pieceso that the measurement light is reflected at the first irradiationposition and then is reflected at a second irradiation position ofanother one of the wave molten solder and the work-piece, and an imageprocessing device disposed on the side of the contact portion fordetecting the first and second irradiation positions as an image date;and the adjustment device adjusts one of a height position of thework-piece and a wave height of the wave molten solder so that aninterval between the first and second irradiation positions becomesconstant.